An ACSM file isn't a book - it's a "key file" which tells Adobe Digital Editions (ADE) how to download a book.
Load the ACSM file into ADE on your PC. It will automatically "trigger" the download of the ePub book. Once it's downloaded, it will be in the "My Digital Editions" folder in "My Documents" on your PC, from where you can drag and drop it into Calibre. You'll need to have the (ahem!) "Tools" installed in Calibre to remove the DRM.
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